Only you can decide this. An old car that has been kept up and is still in good shape may well be worth another semi-costly maintenance expense. If you know the car and still like driving it and the expense is still less than payments on a newer one then possibly. However, if you are not sure of the car's overall condition, if you have been ignoring routine maintenance then no. Timing belt and related are routine. Brakes, steering (control arms, tie rod ends, etc.), tires, trans service, etc. are routine.
If you do not have a good Audi indy shop that services this, or if you are not a DIY then it is time. Otherwise, these cars can go 300,000+ easily.